[develop] ♻️ Refactored code for better testability and added tests 🧪

[jsnel]

Targetting develop branch from which PR #5 was opened targeting main.

• ♻️ Refactored several functions to improve readability and maintainability
• ♻️Replaced os module with pathlib for file operations 👌
• 📚Improved docstrings for clarity and consistency and satify linters 👌
• ♻️ Renamed 'widget' to '_widget' in widget.py to facilitate unit testing 🧪
• 🧪Added a new test case for the _simulate function in widget.py -

[jsnel]

To evaluate this PR, place the attached (zipped) notebook, in the examples folder.
getting_started_with_pyparamgui.zip

This is indeed why I have added these lines to .gitignore:

examples/
!examples/*.ipynb

I found it handy to track changes locally, but they can also be removed again.

We can decide whether or not to add an example notebook later, or just add something to the docs.

[jsnel]

Note the rename of the widget instance to _widget. If you rename this back you will run into the following error during testing:

FAILED tests/test_simulate.py::test_simulate - ZeroDivisionError: float division by zero

The reason for that is that during the (simulate) test, it will use an uninitialized instance of widget, which has all zero or empty values. Renaming the variable avoids this, but it would be better if no global (at import time) instantiation of the widget instance is needed.

Also ideally the widget observer wouldn't need to be initialized separately (as in the previously attached example notebook) but is part of the init.